The use of photoluminescent polymers has attracted significant interest. The potential use of π-conjugated semiconducting polymers in light emitting diodes holds a great potential. Semiconducting polymers combine the processability and outstanding mechanical properties of polymers with the exceptional, readily tailored electronic and optical properties of functional organic molecules.
Poly(phenylenevinylenes) (“PPV's”) represent the most extensively studied class of π-conjugated semi-conducting polymers. Research on these polymers has shown that the molecular and supramolecular architectures of the PPV's affect their electronic properties. Various examples have been shown that the properties may be tailored to many different applications.
A number of low molecular weight oligo(p-phenylenevinylene) derivatives (“OPVs”) have also recently been investigated. The electronic properties of the OPV's have served as models for the corresponding PPV derivative. The substitution of OPV's has resulted in a bathochromatic shift of their emission spectra.
The emission characteristics of the OPV's strongly depend on the state of the matter. Bathochromatic shifts of up to 100 nm have been reported between the emission of the crystalline solid with the emission of a low viscosity molecular solution of the OPV. It has been suggested that the strong π-π overlap encounter in the lattice of the crystalline solid results in the bathochromatic shift. The pronounced π-π interactions cause multiple conjugated molecules to adopt a ‘sandwich structure’ and arrange under cofacial π-π stacking with distances between the planes of the conjugated systems in the order of 3-4′. This supramolecular architecture leads to the formation of excimers that may exhibit strongly red shifted fluorescence bands and long fluorescence lifetimes.